Technical Field
The present invention relates to a communication load determining apparatus that detects the occurrence of abnormality in a communication load in a system in which a plurality of communication devices are connected to a common bus and perform intercommunication via the bus.
A communication system based on a multiple access method is well known. In such a system, a plurality of communication devices are connected to a common bus and perform intercommunication via the bus. With this configuration, each communication device can cooperate with other communication devices to carry out various processes.
Related Art
An example of such a communication system may be an in-vehicle communication system (in-vehicle LAN (local area network)) to which the CAN (controller area network) protocol based on CSMA/CA (carrier-sense multiple access with collision avoidance) scheme is applied.
In a system applied to the CAN protocol, a frame has a header portion in which an identifier (ID) is set for the identification of the communication device as an original transmitter, or the contents of data that are set in a subsequent area. Each of the frames is provided with such an ID. Such a system is ensured to perform arbitration control according to the CAN protocol. Under the arbitration control, when a plurality of frames are concurrently transmitted onto the bus, a right of transmitting a frame (transmission right) is given to the communication device that has transmitted a frame with an ID code of a minimum logical value.
An in-vehicle communication system includes a connector that connects the system to an external tool (hereinafter referred to as external-connection connector). Such an external tool diagnoses conditions of the vehicle or rewrites various programs of electronic control units (ECUs) which are connected to the in-vehicle LAN (i.e. reprograms ECUs). Usually, the connector is a general-purpose standard product and hence can be connected to various machines, such as mobile phones or computers.
Therefore, there is a concern that a third party may maliciously connect some sort of machine to the in-vehicle system via the external-connection connector to produce degradation of the processing functions of the in-vehicle system.
For example, a fraudulent device which, once connected to the external-connection connector, continues transmitting a frame (hereinafter referred to as dummy frame) allocated with an ID code with a minimum logical value may be connected.
When such a fraudulent device is connected, the dummy frame prevents transmission of the frames that should originally be transmitted to the in-vehicle LAN. As a result, the communication load of the in-vehicle LAN becomes extraordinarily large and may thereby cause abnormality in the behaviors of the vehicle.
Thus, if variation in the communication load can be detected, the detection may be used for determining the occurrence of abnormality in the communication load and further for determining connection of a fraudulent device.
As one example, JP-A-2008-211644 discloses an apparatus for detecting variation in a communication load. In the apparatus, an amount of data (number of bits) that are received in a unit time, for example, is detected as a communication load of a bus.
However, in the apparatus disclosed in JP-A-2008-211644, a measuring device (any ECU or a device dedicated to measurement) used for measuring the communication load of the bus is required to receive all of data that appear on the bus and calculate a total amount of received data in a predetermined measurement period. For this purpose, the measuring device is required to ensure a memory capacity that can receive all data, without omission, in the measurement period. Alternatively, if a sufficient memory capacity cannot be ensured, the measuring device is required to complete calculation of an amount of data of each received communication frame, for sequential addition, before receiving the subsequent communication frame. As a result, the CPU that configures the measuring device suffers from a large load. Further, a memory having a large capacity is required to be prepared as the memory that configures the measuring device.
In addition, in the above technique, as the measurement period becomes longer, higher measurement accuracy will be achieved. For this reason, as the measurement accuracy is attempted to be more enhanced, the problems set forth above become more prominent.
These problems are common to communication systems that perform communication via a common bus, not being limited to such in-vehicle LANs.